Molten salt nuclear reactors are based on a critical mass of a fissile material dissolved in a molten salt. This is commonly referred to as fuel salt. They were pioneered at the Oak Ridge National Laboratory in the 1950's to 1970's but have never been successfully commercialised. They have several potential advantages over other reactor types which include the ability to breed fissile 233U from thorium, production of much lower levels of transuranic actinide waste than uranium/plutonium reactors, operation at high temperatures, avoidance of accumulation of volatile radioactive fission products in solid fuel rods and much higher burn up of fissile material than is possible in conventional reactors.
Two major factors have prevented the commercialisation of such reactors.
Many designs of molten salt reactors require attached reprocessing plants to remove fission products continually from the fuel salt. This is necessary since fission products act as neutron poisons, especially in moderated reactors based on a thermal neutron spectrum. It is also necessary to remove insoluble fission products which would otherwise foul pumps and heat exchangers. Such reprocessing plant is complex, expensive and requires extensive development work.
Secondly, molten salts are highly corrosive. While nickel based superalloys are more resistant to such corrosion than standard steels, over long time periods corrosion would still occur. Thus design and manufacture of essential components such as pumps and heat exchangers represents a major development challenge. In principle, new composite materials based on carbon and/or silicon carbide have the chemical resistance to withstand the molten salt but building complex structures such as pumps and efficient heat exchangers from such materials remains very challenging.
Recently, Mattieu and Lecarpentier (Nuclear Science and Engineering: 161, 78-89 (2009)) showed that a non-moderated molten salt reactor could run for a decade or more without reprocessing. Their design still however involved pumps and heat exchangers and could only be built after major research and development of materials for such components.
A critical factor in any molten salt fuelled reactor is extraction of the heat produced by nuclear fission from the fuel salt. Many ways have been proposed to achieve this, a particularly good summary is provided by Taube (1978) (EIR Bericht no 332, Fast reactors using molten chloride salts as fuel). The methods described are                Pumping a molten coolant such as lead, mercury or a volatile salt into the fuel salt so that the coolant both mixes and extracts heat from the fuel salt        Pumping the fuel salt through an external heat exchanger        Pumping a second molten salt or other coolant through pipes passing through the fuel salt with the fuel salt being forcibly pumped in a circulation pattern around the coolant pipes        
All of these proposed designs, other than the first, require pumping of the molten salt in some way. The first design, direct contact between the fuel salt and coolant, has been extensively investigated and is considered impractical for a number of reasons including entrapment of fuel salt in the coolant liquid.
A further design of molten salt reactor was proposed by Romie and Kinyon (ORNL CF 58-2-46, 1958) where the molten fuel salt was allowed to circulate through a heat exchanger by natural convection. This design however allowed only low power output and required a high volume of fuel salt outside the critical area of the core. Large volumes of fuel salt outside the core result in most delayed neutrons being emitted outside the critical area of the core. The resulting low delayed neutron fraction in the critical area of the core renders it unstable and liable to undergo a rapid and uncontrollable increase in power level leading to explosive destruction of the reactor.
A common feature of many conventional non molten salt reactor designs is to place the fuel material passively in tubes, around which coolant circulates, usually by pumping but sometimes just by natural convection. The fuel in the tubes can be a solid, as in the current generation of pressurised water reactors, a paste of solid material in molten sodium (GB 1,034,870), a metal (U.S. Pat. No. 3,251,745) or an aqueous solution (U.S. Pat. No. 3,085,966). Such an arrangement using molten salt fuel was considered by the Aircraft Reactor Experiment (The Aircraft Reactor Experiment-Design and Construction, E. S. Bettis et al, Nuclear Science and Engineering 2, 804, 1957). However, the researchers concluded that it would require fuel tubes with a very small diameter (of the order of 2 mm) in order to prevent overheating of the fuel salt due to the low thermal conductivity of the fuel salt. As a result, the project adopted a system of pumping the fuel salt rapidly through heat exchangers so that the resulting turbulent flow allowed effective heat transfer from the fuel salt to the walls of the much larger tubes. All molten salt reactor designs since then, including the Molten Salt Reactor Experiment which was actually built and operated (ORNL 5011 Molten Salt Reactor Program Semi-annual Progress Report August 1974) have used a similar pumped fuel salt arrangement.